major updates on background, home overview, etc

Author: rcoreilly

citedrefs.json

@@ -0,0 +1,21 @@
++++
+Name = "Copenhagen"
+bibfile = "mechphys.json"
++++
+
+The **Copenhagen** interpretation of quantum mechanics (QM) was developed by Niels Bohr and Werner Heisenberg in the 1920's, and it remains the dominant interpretation among working physicists to this day ([[@Tegmark98]]; [[@SchlosshauerKoflerZeilinger13]]). 
+
+Central to this interpretation is the notion that the physical world operates in two complementary modes: you are _either_ making a _measurement_, which causes the wave function to _collapse_ down to a single discrete particle-like point (via the **Born rule**), _or_ physics is otherwise evolving according to the wave function, which critically preserves all the quantum uncertainty, and just rotates it around in a _unitary_ manner over time.
+
+During this wave-function mode, the mathematical picture suggests that there is no definitive underlying state of the world: everything is in some kind of probabilistic superposition of possible states. Only once you measure something does it actually exist in any kind of definite way, leading to the mantra that "the world only exists when you measure it".
+
+This strong discretization of the laws of physics is at the root of many seeming paradoxes and puzzles in understanding the quantum world: what exactly defines a "measurement" at a fundamental level? How can the wave function, which could conceivably spread out over large macroscopic spaces over time, instantaneously collapse down to a single point within that entire space? 
+
+Despite these conceptual difficulties, the mathematics of the framework allow straightforward calculations that match the outcomes of actual experiments, leading to a general attitude of "shut up and calculate": don't bother with unnecessary considerations of the actual underlying physical ontology, just do the math! This clearly puts this framework into the category of a _calculational tool_ as discussed in [[tools vs models]].
+
+From a sociological perspective, the inability to properly appreciate the distinction between calculational tools and physical models has resulted in a century of the single most egregious form of "gaslighting" in any area of science. New students are trained that their own intuitions about the nature of the physical world are just "naive" biases inherited from the normal macroscopic realm, and that none of these things apply to this mysterious quantum world. You are just supposed to discard all such notions of what is physically plausible, and let the math tell you what is actually going on.
+
+The whole thing is just so impossibly preposterous as to be hilarious, except that generations of serious people have somehow convinced themselves to swallow these absurdities. And they make sure to enforce the dogma on everyone else too: it is truly a cult-like dynamic, right at the heart of the most foundational branch of science.
+
+The [[pilot-wave]] framework, while still incomplete, provides such a striking contrast to the paradoxes present in the Copenhagen interpretation, and yet it remains a "fringe" theory. Sure, if you're actually needing to run some calculations, go ahead and use the [[Hilbert space]] formalism. But if you want to understand what might actually be going on in the underlying physics of Nature, the Copenhagen model is just obviously incoherent and nonsensical.
+

content/copenhagen.md

@@ -14,3 +14,11 @@ Surprisingly, one still observes the interference effect in this case ([[#figure
 
 The [[pilot-wave]] framework of de Broglie and Bohm provides the most natural, intuitive explanation of these effects: the wave goes through both slits, and the particle goes through one, but it is influenced by the wave.
 
+{id="figure_double-slit-deb" style="height:20em"}
+![Trajectories for particles in the double-slit experiment computed according to the de Broglie-Bohm pilot-wave model. The interference effects can be seen as relatively localized bumps in the trajectories, corresponding to steep gradients in the Schrodinger wave equation. Critically, the underlying trajectories are considered to exist at all points even if you don't happen to observe them.](media/fig_double_slit_debroglie_bohm.png)
+
+[[#figure_double-slit-deb]] shows what the underlying trajectories of particles under the pilot-wave framework look like in a double-slit experiment, and [[#figure_double-slit-kocsis]] shows some recent data from an experiment where _weak measurements_ that minimally disturb the system allow one to infer particle trajectories, which look remarkably similar to those predicted by the pilot-wave model ([[@KocsisEtAl11]]).
+
+{id="figure_double-slit-kocsis" style="height:20em"}
+![Reconstructed trajectories of photons in a double-slit experiment using a weak measurement technique that allows aggregate trajectory information to be reconstructed over many repeated samples that are post-sorted according to a weak additional modulation of the system --- these are not individual particle trajectories. There is a striking correspondence to the predictions of the de Broglie-Bohm model. Figure from Kocsis et al, 2011.](media/fig_double_slit_kocsis_et_al_11.png)
+

content/double-slit.md

@@ -2,17 +2,19 @@
 bibfile = "mechphys.json"
 +++
 
-The distinction between **epistemic** vs **ontic** (also known as _aleatoric_ in other contexts) uncertainty is critical for understanding the difference between the standard interpretations of QM (e.g., the [[Hilbert space]] approach) and the [[pilot-wave]] approach.
+The distinction between **epistemic** vs **ontic** (also known as _aleatoric_ in other contexts) uncertainty is critical for understanding the difference between the standard interpretations of QM (e.g., the [[Copenhagen]] interpretation and [[Hilbert space]] approach) and the [[pilot-wave]] approach.
 
 Epistemic uncertainty reflects our own _lack of knowledge_ about the true underlying state of the system, but, critically, excludes any actual "true randomness" arising from the stochastic behavior of the system itself, that would obtain even if we had (counterfactually) perfect knowledge of the underlying state of the system. This latter type of uncertainty is the ontic ("ontologically real") or aleatoric (derived from the latin word for dice) variety.
 
-If the quantum wave function is largely (or even partially) reflecting epistemic uncertainty, then it seriously challenges the pilot-wave framework in a way that does not affect the purely probabilistic Copenhagen approach. How would it make any sense for an _epistemic_ wave of uncertainty to be guiding the _real_ physical positions of particles as they move about the world?
+If the quantum wave function is largely (or even partially) reflecting epistemic uncertainty, then it seriously challenges the pilot-wave framework in a way that does not affect the purely probabilistic [[Copenhagen]] approach. How would it make any sense for an _epistemic_ wave of uncertainty to be guiding the _real_ physical positions of particles as they move about the world? 
 
 By contrast, the Copenhagen interpretation already takes a laissez-faire epistemic-level approach to the wave function in the first place: it is all just a big untouchable ball of mystery until you do a measurement anyway, so it might as well be epistemic or whatever! The Quantum Bayesianism (QBism) approach takes this to its logical extreme, with an entirely subjective epistemic treatment of the wave function ([[@FuchsMerminSchack14]]; [[@Mermin18]]).
 
-Critically, there is clear evidence from _within the pilot-wave approach itself_ that a not-insignificant portion of the pilot-wave actually does represent epistemic uncertainty, because many different possible initial starting states must be modeled to capture our very real uncertainty about the precise starting state of any actual experimental configuration.
+Fortunately, [[@PuseyBarrettRudolph12]] have shown that a purely epistemic account contradicts quantum theory, so there is good reason to believe in the central premise of **wave reality** (see also the [[double-slit]] experiment).
 
-The Heisenberg uncertainty principle dictates that there is a fundamental limit to which we can simultaneously determine all of the relevant degrees of freedom about a physical system, and in practice we almost certainly have well less certainty than this lower limit, because it is very difficult to make any kind of precise measurement of microscopic quantum-scale systems.
+Nevertheless, there is clear evidence from _within the pilot-wave approach itself_ that a not-insignificant portion of the pilot-wave actually does represent epistemic uncertainty, because many different possible initial starting states must be modeled to capture our very real uncertainty about the precise starting state of any actual experimental configuration.
+
+The Heisenberg [[uncertainty principle]] dictates that there is a fundamental limit to which we can simultaneously determine all of the relevant degrees of freedom about a physical system, and in practice we almost certainly have well less certainty than this lower limit, because it is very difficult to make any kind of precise measurement of microscopic quantum-scale systems.
 
 The incorrect incorporation of epistemic uncertainty in the standard Schrodinger pilot-wave framework is also evident in the inevitable spreading out of the wave function over time. In the epistemic case, this spread represents a very sensible increase in uncertainty about where something might be located, given more time since the last time its position was known. But given that the pilot-wave model maintains exact locations of each particle over time, it really doesn't seem to make sense for the wave function to spread out in this manner, at least for variables associated with particle positions.
 

content/epistemic-vs-ontic.md

@@ -51,7 +51,7 @@ $$
 
 Thus, any attempt to decrease the uncertainty in location $\sigma_x$ necessarily increases the uncertainty in momentum $\sigma_p$. This can be seen as a natural consequence of [[matter waves]], and more generally, because a wave is a spatially distributed thing, it is very hard to pin down precisely.
 
-From this complementarity principle, Bohr and Heisenberg developed the _Copenhagen Interpretation_ of QM in the late 1920's, and this is still dominant today. Central to this interpretation is the notion that the physical world operates in two complementary modes: you are _either_ making a _measurement_, which causes the wave function to _collapse_ down to a single discrete particle-like point, _or_ physics is otherwise evolving according to the wave function, which critically preserves all the quantum uncertainty, and just rotates it around in a _unitary_ manner over time.
+From this complementarity principle, Bohr and Heisenberg developed the [[Copenhagen]] interpretation of QM in the late 1920's, and this is still dominant today. Central to this interpretation is the notion that the physical world operates in two complementary modes: you are _either_ making a _measurement_, which causes the wave function to _collapse_ down to a single discrete particle-like point, _or_ physics is otherwise evolving according to the wave function, which critically preserves all the quantum uncertainty, and just rotates it around in a _unitary_ manner over time.
 
 During this wave-function mode, the mathematical picture suggests that there is no definitive underlying state of the world: everything is in some kind of probabilistic superposition of possible states. Only once you measure something does it actually exist in any kind of definite way, leading to the mantra that "the world only exists when you measure it".